The present invention relates to deethanization and ethylene/ethane splitting fractionation steps of cracked gases for olefin recovery. In order to properly appreciate the technological field of the fractionation trains used for separation of olefins from other components in cracked gas, the article "Ethylene from NGL feedstocks--Part 3 Flow Scheme Comparison" (K. Ng et al, Hydrocarbon Processing, December 1983, pp. 99-103) is referred to herein to describe the three most typical choices for the first fractionation step in the fractionation train. A front-end demethanizer, deethanizer and depropanizer are evaluated for their advantages in fractionating cracked gas from NGL feeds. The front-end deethanizer was found, under the assumptions made at the time of the article, to be the most preferable of the fractionation trains.
The article "Ethylene from NGL feedstocks--Part 4 Low Pressure C2 Splitter" (H. Z. Kister et al, Hydrocarbon Processing, January 1984) describes an optimized fractionation step required in olefins separation of cracked gas. The low pressure ethylene/ethane splitter ("C2 splitter") is preferred for the potential for heat pumping the column and therein provides an open ethylene refrigeration loop for other refrigeration needs in the fractionation train. The C2 splitter has been the focus of much study to reduce the relatively expensive utilities required for separation of ethylene and ethane, which have relatively close boiling components.
Other concepts in the prior art that relate to the present invention are described below.
U.S. Pat. No. 1,735,558 describes a multiple sidedraw column crude oil fractionation column. The vapor from three sidedraws from a first column is partly condensed and is condensed and rectified in a second column. The liquid of the second column is returned to the first column for stripping.
U.S. Pat. No. 1,954,839 describes a distillate rectification in which the feed is partly vaporized and the vapor and liquid phases are separated three times to provide for multilevel feeds to a fractionation column. The liquid separated from the last of the partial fractionation stages is recovered as the distillate product.
U.S. Pat. No. 1,957,818 describes rectification of light hydrocarbons and mentions ethylene and ethane as among those. In a series of refluxed and stripped columns, the patent describes using a condensed, rectified overhead stream as feed to a next column. A stripped bottoms stream of the next column is fed to the rectification section of the first column.
U.S. Pat. No. 2,327,643 describes a two column, dual pressure fractionation method wherein the column pressures are chosen to accommodate vaporization of the condensed overhead stream of a second column acting to indirectly supply part of the condensing duty for the overhead stream of a first column. The vaporized second column vapor is recompressed and is fed to the bottom of the first column to supply reboiling duty for the first column.
U.S. Pat. No. 4,285,708 describes a two column method for a single deethanization using a split feed concept. The gaseous feed stream to the deethanization is split and a portion is condensed and stripped in a. stripping column. The overhead vapor from the stripping column is partially condensed and fed to the rectification section of the deethanizer column. The stripping duty of the deethanizer column and the rectification section diameter are substantially reduced by use of an upstream stripping column.
U.S. Pat. No. 4,436,540 describes a full fractionation train for olefin recovery from cracked gas using only low pressure rectification columns for gaseous portions of the pyrolysis furnace effluent. Liquid portions of the rectification columns are further fractionated in high pressure refluxed and stripped columns to complete the separation. Partial intercondensation by pumparounds and liquid streams from the high pressure columns provide rectification duty to the rectification columns.
U.S. Pat. No. 4,720,293 describes a method of feed conditioning to a demethanizer for an olefins fractionation train. The fractionation train's first separation column is the demethanizer, and the feed to it is treated in a dephlegmator to recover ethylene. Column 100 describes a pasteurizing section accommodating removal of residual hydrogen from an overhead ethylene product.
U.S. Pat. No. 4,900,347 describes a system of multiple dephlegmations integrated into a demethanization of an olefins recovery stream. The multiple rectifications in three dephlegmators produce three liquid bottoms streams that are fed to two refluxed demethanization columns. A dephlegmated portion of the feed gas is fed to a second demethanizer column. The overhead product of a first demethanizer is also fed to the second demethanizer. The bottom product of the second demethanizer is a relatively pure stream of ethylene.
U.S. Pat. No. 5,035,732 describes a system similar to that of U.S. Pat. No. 4,900,347, although the second demethanizer is operated at low pressure.
U.S. Pat. No. 5,253,479 describes forming a product specification liquid stream of ethylene as a bottom product of a demethanator column, wherein a portion of the ethylene stream is used as an absorbing, lean liquid in an absorber column. The gas feed to the bottom of the absorber column is the gaseous portion of a partially condensed cracked gas stream comprising at least hydrogen, methane, ethylene and ethane. The absorbing-liquid ethylene and its captured components are fed to a deethylenization column, from which the overhead vapor stream is fed entirely to the demethanator column. It is an apparent disadvantage of the patent process wherein the ethylene condensed at considerable cost in utility must be vaporized in the deethylenizer and recondensed in the demethanator.
In the article "Temperature-Heat Diagrams for Complex Columns, 2. Underwood's Method for Side Strippers and Enrichers" (N. A. Carlberg et al, Ind. Eng. Chem. Res., vol. 28, pp. 1379-1386, 1989), complex columns are described as having benefits and disadvantages. On page 1385, the authors state, "The question to ask is how do complex columns compare against simple column sequences in terms of utility consumption. The answer is that complex columns are more energy efficient but have larger temperature ranges than simple column sequences. Basically, complex columns are more favorable with respect to first-law effects and less favorable with respect to second-law effects. Thus, if there is an adequate temperature driving force, complex columns will be favored; if not, simple columns are more favorable from a utility point of view." A method is presented in the article for evaluating minimum reflux for complex column, i.e. those with one or more side strippers or enrichers. In the article, the operational definition of a side stripper or enricher is a device that withdraws from a column a sidestream vapor or liquid and returns to the same stage a stream comprising liquid or vapor generated in a second column. Side stripping or enriching necessarily returns to the fractionation column a portion of the withdrawn stream which has been enriched or stripped of its original components.
It will be apparent from the above that a simplified and relatively inexpensive method for reducing the combined condensation duties of a demethanizer--deethanizer--C2 splitter combination has not been previously developed. It is an object of the present invention to make such an improvement.